The present invention relates to elevator systems. More particularly, the invention relates to an apparatus for testing aramid fiber cables used in elevator systems in order to determine when such cables are in disrepair and in need of replacement.
Traditional steel elevator cables can easily be visually inspected for wear. The individual wires will break and these breaks can be easily observed. Aramid fiber elevator cables are covered with a protective sheathing that makes visual inspection impossible. If the ropes were not sheathed, it would still be difficult to determine the proper time to replace the ropes because the appearance of the fibers is almost identical whether the fibers are new or in need of replacement.
U.S. Pat. No. 5,834,942, to De Angelis, issued Nov. 10, 1998, discloses an apparatus for determining when a synthetic fiber cable (such as an aramid cable) for an elevator is ready for replacement. The apparatus includes a voltage detection device for detecting a voltage in at least one carbon fiber of the synthetic fiber cable and at least one threshold device for determining when the detected voltage exceeds a predetermined voltage threshold. The detected voltage is dependent upon the integrity of the portion of the synthetic cable (in particular the carbon fibers therein). Exceeding the predetermined voltage threshold is indicative of a failure of the portion of the cable. The prior art device has resorted to placing conductive fibers within the cable so that the fibers can be monitored by electrical means. This apparatus, therefore, may not be suitable for synthetic cables that are not readily conductive.
Those skilled in the art of aramids have shown that the elastic properties of aramid materials can be determined from the measurement of wave propagation through the material. (See M. Ferreira et al., xe2x80x9cNondestructive Testing of Polyaramide Cables by Longitudinal Wave Propagation: Study of the Dynamic Modulusxe2x80x9d, Polymer Engineering and Science, Vol. 40, No.7, July 2000). In particular, it has been observed that polyaramide cables at different states of fatigue have their own speed of longitudinal propagation of acoustic waves. It has been observed that longitudinal waves travel through aramid fiber ropes in accordance with the following formula:                               V          2                =                  E          ρ                                    (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
where V=velocity of wave propagation, E=dynamic or sonic modulus, and xcfx81=density. Since tensile modulus and acoustic modulus both change at the same rate with fatigue it is possible to calculate tensile modulus from the observed values of wave propagation. Plotting E (modulus) against Fr (residual strength), it was found that E=f(Fr). In other words, a quantifiable relationship exists between modulus (determined from velocity) and residual strength.
A similar relationship between modulus and residual strength may be determined for aramid cables used in elevator systems. The relationship will vary based on the particular aramid material used in and the dimensions of the cable. Once the relationship is determined, it will be possible to extrapolate the residual strength from determinations of modulus. This has not heretofore been achieved for elevator systems.
Thus, it is an objective of the present invention to provide an apparatus for inspecting aramid fiber elevator cables and for calculating the residual strength of such cables to determine when they need replacement.
The present invention concerns an apparatus for inspecting and calculating the residual strength of an aramid fiber cable driving an elevator to determine when such cable is in need of replacement. The apparatus includes a transmitter for introducing an acoustic wave that will travel along the aramid fiber cable and a receiver for receiving the acoustic wave after its has traversed a designated section of the cable. The transmitter and receiver provide signals indicating the times the wave was sent by the transmitter and thereafter received by the receiver.
The invention provides a means for processing the first and second signals to calculate the residual strength of the cable. In particular, the invention provides an elevator control system connected to the transmitter and receiver. The control system has a program and associated algorithms that calculate the velocity of the wave based on the times of the first and second signals. The program then calculates the modulus of the cable, and in turn determines the residual strength of the cable. The determination of residual strength is based on a stored equation showing the residual strength as a function of the modulus. The stored equation will vary depending on the particular aramid cable being used in the system.
The transmitter and receiver may be disposed at different locations along the cable. However, in a preferred embodiment, the transmitter and receiver are disposed at the same location along the cable and are placed at a nominal distance from a sheave used in the elevator system. In this case, the velocity of the wave is calculated by measuring the time it takes for the wave to travel from the transmitter to the sheave and back to the receiver. In this embodiment, the transmitter and receiver may actually be contained within one unit.
The invention also concerns an elevator system incorporating an apparatus for inspecting the aramid cable used to drive the system. The elevator system typically comprises an elevator car, an aramid fiber cable connected to the car, a hoist machine having a drive motor for displacing the cable in order to move the car, one or more sheaves for guiding the displacement of the cable, and a counterweight coupled to cable for counter-balancing the weight of the car. The apparatus of the invention is incorporated into the system.